Feedback control for a continuous mixer having a control of internal pressure

ABSTRACT

This disclosure is directed toward an electromechanical feedback control for automatically regulating the discharge temperature in a continuous mixer. The feedback control senses both the mixer discharge temperature and the rate of change of mixer rotor torque. The torque rate of change is detected by a derivative circuit and serves to offer an anticipatory signal to the control system. The feedback control then regulates the internal pressure of the mixer in accordance with an error signal representing the difference between the measured mixer discharge temperature plus torque derivative signals and a preset desired temperature command signal.

United States Patent n91 Notte et a1.

[ Apr. 17, 1973 a [54] I FEEDBACK CONTROL FOR A CONTINUOUS MIXER HAVINGA CONTROL OF INTERNAL PRESSURE {75] Inventors: Angelo Joseph Notte;Peter Hold,

both of Milford; Klaus Juergen Spitzner, Oxford, all of Conn.

3,237,241 3/1966 Gagliardi ..259/192 3,239,878 3/1966 Ah1efe1d.........259/192 3,371,386 3/1968 Ludwig ..2S9/191 7 Primary ExaminerRobertW. Jenkins Att0rneyCharles B. Spencer [57] ABSTRACT This disclosure isdirected toward an e1ectromechanical feedback control for automaticallyregulating the discharge temperature in a continuous mixer. The feedbackcontrol senses both the mixer discharge temperature and the rate ofchange of mixer rotor torque. The torque rate of change is detected by aderivative circuit and serves to offer an anticipatory signal to thecontrol system. The feedback control then regulates the internalpressure of the mixer in accordance with an error signal representingthe difference between the measured mixer discharge temperature plustorque derivative signals and a preset desired temperature commandsignal.

2 Claims, 5 Drawing Figures [52] US. Cl ..259/10, 259/191 [51] Int. Cl...B0lf 7/02 [58] Field of Search ..259/l91, 192, 9, 259/10, 25, 26, 45,46; 425/149, 145, 143;

[56-] References Cited UNlTED STATES PATENTS 3,154,808 11/1964 Ahlefeld..259/192 FEEDBACK CONTROL FOR A CONTINUOUS MIXER HAVING A CONTROL OFINTERNAL PRESSURE This invention relates generally to continuous mixerssuch as are disclosed and claimed by the Ahlefeld Jr. et al. U.S. Pat.No. 3,154,808 dated Nov. 3, 1964. Such a continuous mixer includes amixing enclosure having interspaced entrance and exit openings andcontaining mixing means which cannot alone force material through thisenclosure but does permit material to be forced through this enclosureat a rate dependent on the rate at which material is forced into theentrance opening to displace or push material already in the enclosureto the exit opening and out through the latter.

Control of the internal pressure of the material being mixed in such amixer is effected by variably restraining the discharge of mixedmaterial displaced or forced from the mixer when unmixed material is fedinto the mixer.

The control disclosed and claimed by the Ahlefeld et al. patentworks onthe general principle of a fluid flow choke using static surfaces overwhich the material flows and operates by varying the cross-sectionalarea of the opening through which the material is discharged. Control ofsuch a discharge restraining means automaticallyin response to thetemperature of the discharge material, is disclosed by the GagliardiU.S. Pat. No. 3,237,241 dated Mar. 1, 1966. The

discharge temperature is a function of the pressure on the materialbeing mixed in the mixer.

An improvement internal pressure control provides the material dischargewith one or more elements which engage the discharge flow and aremovable in the directionof flow at a speed controllable to providerestraint to this flow. Examples are disclosed and claimed by theScharer et al. patent application Ser. No. 91,351 filed Nov. 20, 1970.

In a continuous mixer such as is disclosed and claimed in the Ahlefeldpatent, which utilizes either of the two aforementioned internalpressure controls, a critical process parameter is the temperature ofthe discharged material. This temperature is controlled by adjusting theback pressure applied to the material during discharge. The backpressure, in turn, varies the amount of mixing energy absorbed by thematerial from the mixer rotor. The .effect of a change in the amount ofmixing energy absorbed by the material (i.e. as measured by the changein power applied to the mixer rotor) is a corresponding change in thesteady state temperature of the material discharged from the mixer.

In the Gagliardi Patent cited above, the back pressure of the materialbeing mixed. is controlled in response to the temperature of thedischarged material by forming a direct dependence between the backpressure and thedischarge temperature. HOwever, since the time constantassociated with discharge temperature changes caused by variations inback pressure is several minutes, a control that uses a temperaturefeedback only can result in either extensive sluggishness or severeovershoots. Anticipatory networks applied to temperature, itself, wouldtend to improve this condition but the practical application of thesenetworks requires the use of differentiators which are objectionablebecause of their noise susceptibility and large reactive componentssince extremely low frequencies are involved. The time constantassociated with mixer rotor power or torque changes caused by variationsin back pressure is, however, several orders of magnitude smaller thanthe time constant associated with temperature changes caused byvariations in back pressure. Furthermore, there exists a relationshipbetween this torque change and the steady state temperature of thedischarge material.

The present invention uses the rate of change of mixer rotor torque asone of the feedback signals to control the discharge steady statetempetature. After transients die out, this derivative signal reduces tozero, forcing the ultimate comparison to be made between actualtemperature andthe setpoint. Since the system has a pure integration inits foreward loop, corrective action will occur until the comparison(error) signal drops to zero.

A specific embodiment of the present invention is disclosed below inconnection with the continuous mixer using a screw extruder as anexample of the improvement internal pressure control.

In the accompany drawings:

FIG. 1 is a side view of the extruder in longitudinal section and of themixer in transverse section, both on a vertical plane, with theexception of the extruder screw driving motor which is shown inelevation;

FIG. 2 is a top view of FIG. 1;

FIG. 3 is a cross section taken on the line 33 in FIG. 1;

FIG. 4 schematically shows the power drive for the mixer; and I FIG. 5schematically shows the electrical control system of the presentinvention.

Referring first to FIGS. 1 to 3, the continuous internal mixer is shownas comprising a barrel 1 which may have either of the external formsshown by FIGS. 1 and 3. This barrel forms two parallel, laterallyinterconnecting cylindrical chambers 2 having at one end a commondischarge orifice 3. Bladed rotors 4 are located in the chambers 2. Thematerial to be mixed is stuffed under pressure into the chambers 2 at anentrance location 20 spaced from the discharge orifice 3, a verticalport 5 leading to stuffing screw blades 6 formed on the rotors 4 andwhich push the material through entrance openings 20 of the two barrels.The rotors 4 are intergeared for opposite rotation by gears 7 andsupplied with a rotary power drive system schematically shown in FIG. 4as comprising an electric motor 8 driving through a gear reduction unit8a. Each of the rotors 4 has a blade 9 with a cross section which issubstantially like that of a Banbury-type blade and having a portionthat twists away from its direction of rotation and a portion thattwists in the opposite direction, the length and twist ratio betweenthese oppositely twisting portions of each blade being such that whenthe chambers 2 contain the material stuffed into them the average of theaxially directed forces applied to the material by the blades isinsufficient to force the material through the orifice 3. In this waythe overall axial travel of the material through the chambers 2 isdependent on the rate at which it is received through the port 5. Eachrotor may have more than one blade, each blade has the two portionsreferred to, and although not shown, these portions may be displacedrelative to each other circumferentially with respect to the rotor.

The exit opening or discharge orifice 3 opens transversely from thechambers 2 and at least mainly in overlapping relation with respect tothe portions of the blades 9 adjacent thereto. The blades may have othercontours where they overlap the orifice 3, or separate elements (notshown) may be used to stuff the mixed material through the orifice 3.With opposite rotation of the rotors so that the blades of both turndownwardly in the direction of the orifice 3, there is a force exertedon the material by the blades, or other elements if used, to stuff itthrough this discharge orifice. This escape of the material must berestrained in a controllable manner for the pressure onthe materialbeing mixed in the chambers 2 to be produced and controlled.

As shown by FIGS. 1 and 3 in particular, the discharge orifice 3 isconnected directly and positively to the inlet 10 of a screw-typeextruder having a barrel 11 forming a cylindrical chamber 12 in which ascrew 13 is rotatively positioned. This screw is connected to a variablespeed rotary motor or extruder drive 14 by means of which the rotativespeed of this screw may be positively controlled. The chamber 12 has anoutlet or extrusion orifice 15.

The motor 14 may be of the hydraulically operated type such as one ofthose commercially available from Houdaille Industries, Inc., BuffaloHydraulics Division. However, any motor may be used providing adequatetorque and which can be controlled as to its speed and which can drivethe screw 13 at a selected constant rotary speed.

Adjacent to the extruder's inlet 10 and the mixers discharge orifice 3under operation the pressure in the extruder's chamber 12 is a backpressure, or in other words, a negative pressure with respect to theforce on the material extruded through the outlet or extrusion orifice15. Somewheres between that location and the orifice 15 the pressure onthe material becomes positive with respect to the extruders dischargeorifice, or in other words, the screw will exert a forwardly directforce on the material.

When the patented continuous mixer is started up from its shutdowncondition, it is preferably to close its discharge orifice 3 completelyuntil the material charged has reached a plasticized condition andsubstantially stable operation is obtained. Therefore, as shownparticularly by FIG. 3, a shutoff valve 16 of the sliding type isinterposed between the discharge orifice 3 and the extruders inlet 10.This valve is closed when starting up the mixer but is thereafter openedand not used to restrain or choke the discharge of the mixed material.When the mixer is in operation, the valve is open and it is thescrew-type extruder that provides the restraint to the dischargematerial.

As shown by FIG. 3, the continuous mixer is rigidly supported by a base17 resting on a solid foundation 18, with its barrel 1 and any partsconnected to it projected from this base in cantilever fashion. In thisFIG. 3 the screw-type extruder has its barrel 11 supported immovably bysupports which are fastened to an immovable base member 19. The extruderis positioned transversely with respect to the mixer.

In the operation of the form of continuous mixer shown in FIGS. 1 to 3,material discharged through the discharge orifice 3 of the continuousmixer is engaged by a moving surface or surfaces in the form of theblade or blades of the screw of the screw-type extruder. If the screw isnot rotating, the pressure on the material in the discharge orifice 3 isinsufficient to drive the material through the spiral course or coursesof the screw's blade or blades. As the material is continuously chargedinto the mixer, the pressure on the material inside of the mixerincreases and when this has the desired value the screw should berotated at a rate just sufficient to match the rate at which the mixeris fed.

During operation a slowdown of the screw serves to increase the mixersinternal pressure, after which the screw can be returned to the ratematching the mixer's charging rate. A speedup of the screw drops thepressure in the mixer. In all instances the material should bedischarged from the mixer at substantially the rate at which the mixeris charged with material to be mixed, regardless of the selected mixerinternal pressure.

Automatic control of the screw-type extruder is possible. Thus, FIG. 3shows a temperature sensing element 20, such as a thermocouple,connected by wiring 21 to a control system or control 22 for the motor14 as shown in FIG. 1. The control 22 may be as shown by the previouslymentioned Gagliardi U.S. Pat. No. 3,237,241, but modified to operate theusual speed controller of the hydraulic motor 14.

The present invention is an improvement of the above control and isshown in diagrammatic form in FIG. 5. The control 22 of the presentinvention controls the extruder drive motor 14 not only in response tothe discharge temperature measured by discharge temperature transducer20 and conveyed to the control 22 on wire 21 but also in response to thetorque of the mixer rotor 4 measured on the rotor torque transducer wire23 by a mixer rotor torque transducer 24 and conveyed on wire 25 tocontrol 22 of FIG. 5, as schematically shown by FIG. 4.

Referring to this schematic diagram of FIG. 5, a reference commandsignal is derived on wire 32 from a temperature set point potentiometer34. The temperature set point potentiometer 34 is set to produce asignal on wire 32 which is proportional to a pre-determined temperaturewhich is desired for the discharge material. A signal proportional tothe measured mixer rotor torque of the continuous mixer is derived onwire 25 from the torque transducer 24.

The mixer rotor torque signal is led on wire 25 to integrator 27, and onwire 26 to comparator 35. The integrator 27 output signal is fed back onwire 30 to the input of the integrator 27 and is also led by a wire 28to comparator 35. The combination of integrator 27 and comparator 35forms a compensating network which takes the derivative of the lowfrequency portion of the mixer torque signal and attenuates the highfrequency portion. The output of comparator 35 has therefore a magnitudeonly duringtransients. In the steady state (i.e. when the mixer torqueis not changing) the output of comparator 35 is zero.

The discharge temperature transducer 20 measures the dischargetemperature of the material being discharged from the continuous mixerand generates a signal on wire 21 which is directly proportional to themeasured discharge temperature. The temperature set point potentiometer34 is calibrated such that its output signal on wire 32 will beidentical to that produced at the discharge temperature transduceroutput on wire 21 if the temperature measured by the dischargetemperature transducer matches the desired temperature set at thetemperature set point potentiometer 34.

The discharge temperature on wire 21 is led to a noise filter 40. Theoutput of this filter is led on wire 42 to comparator 50. The other twoinputs of comparator 50 are the temperature set point on wire 32 and thecompensated mixer torque signal on wire 36. Comparator 50 output is ledon wire 52 to integrator 54, and on wire 58 to amplifier 60. Summer 64then combines the wire 56 signal from integrator 54 with the wire 62signal from amplifier 60. The resulting speed command signal is led onwire 66 to the extruder drive 14.

Because of the integrator 54, the error signal on wire 52 will beintegrated in such a fashion as to alter the speed command signal onwire 66 to the extruder drive 14 such that the error signal is reducedto zero. Since, in the steady state, the compensated mixer torque signalon wire 36 is zero, the mixer discharge temperature on wire 42 is forcedto equal the temperature setpoint on wire 32. There is therefore notemperature error in the steady state.

A step command change in discharge temperature can result in a newextruder steady state speed that is higher, lower, or equal to the speedbefore the change was introduced; The rate of change of speed during thetransient state, however, is positively correlated with the mixer torquederivative What is claimed is:

1. Apparatus for controlling the temperature of the discharge materialin a continuous mixer having a mixer rotor for mixing material andhaving a control of the pressure on the material within said mixercomprising: s

means for measuring the discharge temperature of the material to bemixed and producing a signal representing said discharge temperature;

means for measuring the mixer rotor torque and producing a signalrepresenting said torque;

an electrical control circuit adapted to receive the dischargetemperature signal from the discharge temperature measuring means andadapted to receive the mixer torque signal from the mixer torquemeasuring means and to affect the mixer torque signal in such a way asto produce the filtered derivative of the mixer torque;

means for producing a signal representing a predetermined desiredtemperature of the discharge material;

-means for activating the pressure control of said mixer in response toasignal representing the difference between the desired dischargetemperature signal and the actual measured discharge temperature.

2. A continuous mixer comprising a mixing enclosure having interspacedentrance and exit openings, mixing means contained in said enclosure andwhich cannot drive material through this enclosure but does permitmaterial to be pushed through this enclosure while being mixed, meansfor pushing material through said entrance opening into said enclosureand means for restraining the movement of material from said enclosuresthrough said exit opening; wherein the improvement comprises saidrestraining means including one or more surfaces contacted by thematerial moving from said enclosure and which move in the direction ofthe movement of this material and means for controlling the movement ofsaid surface or surfaces to control the restraint on said material andtemperature sensing means located to measure the temperature of the heatcreated in said material by said mixing means, and means for measuringthe power contributed by said mixing means to said material and meansautomatically responsive to said temperature sensing means and saidmixer power measuring means for controlling said means controlling themovement of said surface or surfaces to control the restraint on saidmaterial.

1. Apparatus for controlling the temperature of the discharge materialin a continuous mixer having a mixer rotor for mixing material andhaving a control of the pressure on the material within said mixercomprising: means for measuring the discharge temperature of thematerial to be mixed and producing a signal representing said dischargetemperature; means for measuring the mixer rotor torque and producing asignal representing said torque; an electrical control circuit adaptedto receive the discharge temperature signal from the dischargetemperature measuring means and adapted to receive the mixer torquesignal from the mixer torque measuring means and to affect the mixertorque signal in such a way as to produce the filtered derivative of themixer torque; means for producing a signal representing a predetermineddesired temperature of the discharge material; means for activating thepressure control of said mixer in response to a signal representing thedifference between the desired discharge temperature signal and theactual measured discharge temperature.
 2. A continuous mixer comprisinga mixing enclosure having interspaced entrance and exit openings, mixingmeans contained in said enclosure and which cannot drive materialthrough this enclosure but does permit material to be pushed throughthis enclosure while being mixed, means for pushing material throughsaid entrance opening into said enclosure and means for restraining themovement of material from said enclosures through said exit opening;wherein the improvement comprises said restraining means including oneor more surfaces contacted by the material moving from said enclosureand which move in the direction of the movement of this material andmeans for controlling the movement of said surface or surfaces tocontrol the restraint on said material and temperature sensing meanslocated to measure the temperature of the heat created in said materialby said mixing means, and means for measuring the power contributed bysaid mixing means to said material and means automatically responsive tosaid temperature sensing means and said mixer power measuring means forcontrolling said means controlling the movement of said surface orsurfaces to control the restraint on said material.